1. Field of the Invention
The present invention relates to the field of telecommunications and, more particularly, to a circuit for synthesizing a floating complex impedance across the tip and ring leads of a telephone line circuit.
2. Description of the Prior Art
Telephone line circuits are customarily found in the telephone switching system or central office of a telecommunications network. The telephone line circuit interfaces the central office, to a telephone or subscriber station found at a location remote from the central office. The telephone line circuit functions to supply power or battery feed to the subscriber station via a two wire transmission line or subscriber loop and to couple the intelligence or voice signal to and from the telephone switching system.
In many presently known telephone line circuits the battery feed function has been performed by using a passive, highly balanced, split winding transformer and or inductors which carry up to 120 ma dc. This passive circuit has a wide dynamic range, passing noise-free differential signals while not overloading with the 60 Hz longitudinal induced currents. The line circuit just described, feeds dc current to the subscriber loop and also provides the voice path for coupling the voice signal between the subscriber station and the central office. The electromagnetic components of passive line circuits are normally bulky and heavy and consume large amounts of power for short subscriber loop lengths where the current fed to the subscriber station is more than necessary for equalization. Active line-feed circuits can be less bulky and require lower total power, but meeting dynamic range and precision balance requirements dictates an overly complex circuit design.
Recently, solid state replacements for the electromagnetic components of the aforementioned line circuits have been developed. Devices such as high voltage bipolar transistors and other specialized integrated circuits are being designed to replace the heavy and bulky components of the electromagnetic line circuit. Such a device is described in the IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. SC-16, NO. 4, August 1981, entitled, "A High-Voltage IC for a Transformerless Trunk and Subscriber Line Interface." These smaller and lighter components allow the manufacture of telephone switching systems having more line circuits per circuit card as well as decreasing the physical size of the switching system.
However, presently known solid state line circuits, still suffer from deficiencies in meeting good transmission performance specifications. These deficiencies manifest themselves in poor longitudinal balance and poor longitudinal current susceptibility, which cause the circuit to fail or to become noisy. Other problems presently encountered are excessive power dissipation at short loops that consume prodigious amounts of central office power and 2 wire input impedance circuits that are complex and that exhibit poor return loss.
Accordingly, it is an object of the present invention to provide a new and more effective circuit that will effectively and efficiently synthesize a complex floating impedance across the tip and ring leads of a telephone line circuit matching the impedance of a connected subscriber loop.